Image formation device and image formation method

ABSTRACT

Image data are inputted from a data processing means ( 23 ) into storage means ( 24 ) so that light emitting elements on one line ( 28   a ) of a light emitting element (yellow) line head ( 28 ) are actuated according to signal outputted from a shift resistor ( 24   a ) so as to expose pixels on an image carrier. At a point of time when the pixels reach a position corresponding to the light emitting elements in a next line ( 28   b ) by moving the image carrier in a direction of arrow X, the image data are transmitted to a shift resistor ( 24   b ) and then outputted to the line ( 28   b ) so as to expose the pixels again. The image data are transmitted among the shift resistors sequentially by moving the image carrier, thereby achieving the multiple exposure of the same pixels.

TECHNICAL FIELD

The present invention relates to an image forming apparatus and an imageforming method which are directed to simplify the circuit structure andto speed up the light emitting control during exposure of pixels on animage carrier in multiple exposure manner capable of outputtinggradation.

BACKGROUND ART

In conventional image forming apparatus in which a latent image iswritten on an image carrier, it is known to employ an LED (lightemitting diode) array as writing means. In case of employing lightemitting elements such as an LED, it is necessary to pay attention tothe relation between the luminance (amount of light) and the lifeduration of each light emitting element. That is, the life duration canbe increased by reducing the luminance of the light emitting element. Inthis case, however, there is a problem that the amount of light forexposure is insufficient to form image. When the luminance of the lightemitting element is increased, enough amount of light for exposure forforming image is obtained. In this case, however, there is a problemthat the life duration is shortened.

For this, the development of material for obtaining light emittingelements capable of providing large luminance and having long lifeduration has been encouraged. However, in the present state of affairs,it is too expensive to achieve the practical use. In this connection, aline head (optical head) of multiple exposure type in which each pixelis exposed by a plurality of light emitting elements has been developed.As an example (1) of such line heads of multiple exposure type, JapanesePatent Unexamined Publication No. S61-182966 discloses a recording arrayhead on which light recording elements are aligned in a plurality ofrows in the rotational direction of a photoreceptor drum. An image datais overlappingly recorded at the same pixel by shifting the lightemitting recording elements in the direction of the rows with moving thephotoreceptor drum. The example (1) has an advantage that higher speedimage formation is achieved even using light recording elements with lowlight-emitting output.

Another example (2) is disclosed in Japanese Patent UnexaminedPublication No. S64-26468 in which an EL element panel is composed of ELelement group of 20 dots×640 dots (vertical×horizontal). The EL elementgroup is driven at a speed same as the moving speed of a photoreceptorfor every line. Accordingly, the amount of light irradiated on a singlepixel is twentyfold the amount of light emitted from each EL element.This example also can cope with high speed of image formation becausethe amount of light of exposure per pixel is increased.

Another example (3) is disclosed in Japanese Patent UnexaminedPublication No. H11-129541 and is a print head on which LEDs are alignedin a plurality of lines such that multiple exposure is made on eachpixel by moving the print head in the main scanning direction. In thisexample, since the multiple exposure is conducted, variations in amountof light among the respective LEDs can be equalized, thereby improvingthe image quality. Still another example (4) is disclosed in JapanesePatent Unexamined Publication No. 2000-260411 and is an optical printerhead on which plural lines of LED array chips are aligned. The gradationof each pixel can be changed among three levels by turning ON or OFF theLED array chips on each line.

The aforementioned examples (1) and (2) relate to a technology ofmonochrome image formation and therefore have a problem that gradationcontrol for neutral density is impossible. The example (3) relates to atechnology of a serial type in which the line head is driven andtherefore has a problem of having a complex driving mechanism. Theexample (4) relates to a technology in which the LED array chips on eachline are turned ON and OFF and therefore has a problem of complexity ofthe control circuit.

Since the number of light emitting elements in the line head of multipleexposure type are greater than that of a line head of normal exposuretype and it is necessary to control the light emitting elementssynchronously with the movement of the photoreceptor, there is a problemof complexity of the control circuit for conducting the data processingand it is therefore difficult to achieve higher speed light emittingcontrol. Especially, in case that a line head (optical head) ofmulti-exposure type is employed for color image formation, the amount ofdata to be processed must be severalfold that in the case of ON-OFFcontrol because the gradation control for each pixel is sometimesrequired. This makes the speeding up of emission control furtherdifficult. In case of line head of multiple exposure type, it isrequired to send a large amount of data processed by a data processingunit to the line head, thus increasing the number of wires between theline head and the body of an image forming apparatus. Accordingly, it isnecessary to employ an interface capable of supporting the high-speedprocessing, thus increasing the cost.

The present invention was made in view of the aforementioned problems ofconventional techniques and the object of the present invention is toprovide an image forming apparatus and an image forming method which aredirected to simplify the circuit structure and to speed up the lightemitting control during the exposure of pixels on an image carrier inmultiple exposure manner capable of outputting gradation.

DISCLOSURE OF THE INVENTION

A first image forming apparatus of the present invention achieving theaforementioned object is an image forming apparatus in which a pluralityof lines each having a plurality of light emitting elements are arrangedto have rows in the sub scanning direction of an image carrier so thatlight emitting elements are arranged in a matrix in a plane, whereinpixels on said image carrier are exposed by the light emitting elementsaligned in one line and exposed again by the light emitting elementsaligned in the next line after the movement of said image carrier, andin the same manner, said pixels are sequentially exposed by the lightemitting elements on another line after the movement of said imagecarrier so as to achieve multiple exposure of the pixels. The firstimage forming apparatus is characterized by comprising control means forcontrolling said light emitting elements, which are disposed on therespective lines for exposing a same pixel, to emit the same amount oflight, wherein the pixels can be exposed according to gradation outputformed by said control means.

The first image forming apparatus of the present invention ischaracterized by comprising storage means for storing image data formedby said control means and outputting said image data to said lightemitting elements, wherein said storage means is composed of means whichare arranged to correspond to the lines of the light emitting elements,respectively and are designed to transport image data, hold the imagedata, and output the image data to the light emitting elements. Thefirst image forming apparatus is further characterized in that there arelines of pixels to be exposed and lines of pixels not to be exposed onsaid image carrier, the light emitting elements on the respective linesare arranged to correspond to the lines of pixels to be exposed,respectively, said storage means are arranged to correspond to both thelines of pixels to be exposed and the lines of pixels not to be exposed,respectively, and the storage means corresponding to the lines of pixelsnot to be exposed do not output said image data.

Furthermore, the first image forming apparatus of the present inventionhas the following characteristics with regard to said light emittingelements: (1) the interval in the sub scanning direction between spotpositions formed on the image carrier by the light emitting elements isan integral multiple of the pixel pitch in the sub scanning direction;(2) the light emitting elements are controlled by a driving circuitaccording to the active matrix method; (3) the amounts of light of thelight emitting elements are controlled in the PWM method; (4) theamounts of light of the light emitting elements are controlled in theintensity modulation method; and (5) each of the light emitting elementscomprises an organic EL.

The first image forming apparatus of the present invention is stillfurther characterized in that the image forming apparatus is of a tandemtype which comprises at least two image forming stations each having animage carrier and further having a charging means, an exposure head, adeveloping means, and a transfer means which are arranged around saidimage carrier and forms a color image by passing a transfer mediumthrough the respective stations.

In the first image forming apparatus of the present invention, once thecontrol means forms data for the first one line, the operations of allof light emitting elements in the line head can be controlled by storingthe image data for the first one line in the storage means (shiftresistors) and just transmitting the image data among the storage means.Therefore, the control means is not required to produce data for alllight emitting elements of the line head, thereby simplifying thestructure of circuit and achieving the high-speed data processing.

In the first image forming apparatus of the present invention, thestorage means for pixel lines and the light emitting element lines canbe arranged to correspond to each other. In this case, the timing fortransmitting image data stored in a storage means to the next storagemeans and the timing for making light emitting elements in the line toemit light on the basis on the image data for a pixel line stored in thestorage means can be synchronized, thereby simplifying the circuitstructure and speeding up the operation of the light emitting elementlines.

Further, in the first image forming apparatus of the present invention,the light emitting elements are controlled according to the activematrix method. Accordingly, the light emitting elements can bemaintained to keep emitting light by means of condensers and transistorsarranged around the light emitting elements. Therefore, the lightemitting elements remain to emit light even during the transmission ofimage data from a storage means to the next storage means, therebyexposing pixels with high luminance.

In addition, in the first image forming apparatus of the presentinvention, the amount of light emitted from the light emitting elementsis controlled in the PWM method. Since the amount of exposure can bechanged by ON/OFF control of the light emitting elements, the circuitstructure can be simplified. Moreover, in the first image formingapparatus of the present invention, the amount of light emitted from thelight emitting elements is changed in the intensify modulation method.Therefore, it is not required to control the ON/OFF of the lightemitting elements at a high speed. Even when the speed of response ofthe light emitting elements is slow, the amount of exposure can bechanged at a high speed. In addition, in the first image formingapparatus of the present invention, the light emitting elements can beeasily formed on a glass substrate, thereby achieving lower price.

A first image forming method of the present invention achieving theaforementioned object is an image forming method using a plurality oflines each of which has a plurality of light emitting elements and whichare arranged to have rows in the sub scanning direction of an imagecarrier and using storage means, designed to transport image data formedby control means, hold the image data, and output the image data to thelight emitting elements, which are arranged to correspond to the linesof the light emitting elements, respectively. The first image formingmethod is characterized by comprising a step of actuating the lightemitting elements on the first line to expose pixels on the imagecarrier to light according to the image data outputted from the storagemeans, a step of moving the image carrier for a pixel pitch, a step oftransmitting the image data to the storage means for the next linesynchronously with the movement of the image carrier in timing, and astep of actuating the light emitting elements on the next line to emitthe same amount of light as that of the light emitting elements on theformer line to repeatedly expose said pixels so that said pixels aresubjected to multiple exposure by the light emitting elements on therespective lines by transmitting the image data by the storage meanswith moving the image carrier for the pixel pitch. The first imageforming method is characterized by further comprising a step ofactuating the light emitting elements according to the gradation outputformed by said control means to expose the pixels.

A second image forming apparatus of the present invention achieving theaforementioned object is an image forming apparatus in which a pluralityof lines each having a plurality of light emitting elements are arrangedto have rows in the sub scanning direction of an image carrier so thatlight emitting elements are arranged in a matrix in a plane, whereinpixels on said image carrier are exposed by the light emitting elementsaligned in one line and exposed again by the light emitting elementsaligned in the next line after the movement of said image carrier, andin the same manner, said pixels are sequentially exposed by the lightemitting elements on another line after the movement of said imagecarrier so as to achieve multiple exposure of the pixels. The secondimage forming apparatus is characterized by comprising storage means forstoring information of misalignment of the mounted position of the linehead relative to the apparatus, light emitting elements for adjustingthe image position which are preliminarily arranged in respective linesof said line head, and control means for inserting blank data to theimage data for every line of the light emitting elements correspondingto the misalignment so as to form image in normal position by correctingsaid misalignment of the mounted position of the line head according tothe information of misalignment of the mounted position of the linehead.

According to the second image forming apparatus, it is not required tomechanically correct the misalignment of image forming portions. Thatis, the misalignment in image formed by image forming portions can becorrected by controlling the positions where image data are written,thereby eliminating the mechanical adjustment. Therefore, themisalignment in image can be easily corrected in the line head forconducting multiple exposure.

A third image forming apparatus of the present invention achieving theaforementioned object is an image forming apparatus comprising lineheads in which a plurality of light emitting elements are arranged in amatrix in a plane to form a plurality of unicolor images to besuperposed on each other, storage means for storing information ofmisalignment of the mounted position of the line head relative to theapparatus, and control means for inserting blank data to the image datafor every line of the light emitting elements corresponding to themisalignment so as to form image in normal position by correcting saidmisalignment of the mounted position of the line head according to saidstored information of misalignment of the mounted position of the linehead.

According to the third image forming apparatus of the present invention,even when the mounted position of one of line heads is shifted from thenormal position in an image forming apparatus for forming a color image,the misalignment in image can be easily corrected without moving theposition of the line head.

Further, the third image forming apparatus is characterized in that theimage forming apparatus is of a tandem type which comprises at least twoimage forming stations each having an image carrier and further having acharging means, an exposure head, a developing means, and a transfermeans which are arranged around said image carrier and forms a colorimage by passing a transfer medium through the respective stations.According to the third image forming apparatus of the present invention,in an image forming apparatus of a tandem type, the misalignment inimage can be easily corrected.

A second image forming method of the present invention achieving theaforementioned object is an image forming method using a plurality oflines each of which has a plurality of light emitting elements and whichare arranged to have rows in the sub scanning direction of an imagecarrier and using storage means, designed to transport image data formedby control means, hold the image data, and output the image data to thelight emitting elements, which are arranged to correspond to the linesof the light emitting elements, respectively. The second image formingmethod is characterized by comprising a step of preliminarily arranginglight emitting elements for adjusting the image position respectivelines of said line head, a step of storing information of misalignmentof the mounted position of the line head relative to the apparatus, astep of inserting blank data to the image data for every line of thelight emitting elements corresponding to the misalignment so as to formimage in normal position by correcting said misalignment of the mountedposition of the line head according to said stored information ofmisalignment of the mounted position of the line head, a step ofactuating the light emitting elements on the first line to expose pixelson the image carrier according to the image data outputted from thestorage means, a step of moving the image carrier for a pixel pitch, astep of transmitting the image data to the storage means for the nextline synchronously with the movement of the image carrier in timing, anda step of actuating the light emitting elements on the next line to emitthe same amount of light as that of the light emitting elements on theformer line to repeatedly expose said pixels. Further, the second imageforming method is characterized by further comprising a step ofactuating the light emitting elements according to the gradation outputformed by said control means to expose the pixels.

A fourth image forming apparatus of the present invention achieving theaforementioned object is an image forming apparatus comprising a linehead in which a plurality of lines each having a plurality of lightemitting elements aligned in the main scanning direction are arranged tohave rows in the sub scanning direction of an image carrier so thatlight emitting elements are arranged in a matrix in a plane, whereinpixels on said image carrier are exposed by the light emitting elementsaligned in one line and exposed again by the light emitting elementsaligned in the next line after the movement of said image carrier, andin the same manner, said pixels are sequentially exposed by the lightemitting elements on another line after the movement of said imagecarrier so as to achieve multiple exposure of the pixels. The fourthimage forming apparatus is characterized by comprising storage means forstoring information of tilt of the line head relative to the mainscanning direction, image data supplying means for supplying image datato the respective light emitting elements, delaying means for delayingthe timing of supplying image data from said image data supplying meansto the light emitting elements, and control means for conducting delaycontrol to the image data to be supplied from said delaying means tolight emitting elements according to said information of tilt in such amanner that the position of image formation corresponding to pixels onthe image carrier is corrected from the tilt of the line head.

The fourth image forming apparatus is characterized in that the imageforming apparatus is of a tandem type which comprises at least two imageforming stations each having an image carrier and further having acharging means, an exposure head, a developing means, and a transfermeans which are arranged around said image carrier and forms a colorimage by passing a transfer medium through the respective stations.

The fourth image forming apparatus conducts the following delay controlin order to correct the tilt of the line head: (1) the light emittingelements are divided into a plurality of blocks and the delay control isconducted to image data to be supplied to said light emitting elementsfor every block; (2) a plurality of said line heads are arranged tocorrespond to different colors, respectively, and the light emittingelements of the line head which is tilted is subjected to said delaycontrol during multiple exposure in which the respective colors aresuperposed on each other; and (3) among said plurality of light emittingelement lines, the first light emitting element line is controlled witha delay control signal for correcting the tilt of said line head and thelight emitting element lines including and after the second lightemitting element line are controlled with signals formed by addingsignal corresponding to the timing shift from the former light emittingelement line to the aforementioned delay control signal for the frontlight emitting element line.

The storage means in the fourth image forming apparatus of the presentinvention has the following characteristics: (1) the storage means isdisposed in the apparatus body; (2) the storage means is disposed in acartridge in which the line head is arranged; (3) the storage means isdisposed in the line head.

The light emitting elements in the fourth image forming apparatus of thepresent invention have the following characteristics: (1) said lightemitting elements are controlled by a driving circuit according to theactive matrix method; (2) the amounts of light of said light emittingelements are controlled in the PWM method; (3) the amounts of light ofsaid light emitting elements are controlled in the intensity modulationmethod; and (4) each of said light emitting elements comprises anorganic EL.

In the fourth image forming apparatus of the present invention, if theline heads are installed to the apparatus such that one or more of theline heads is tilted relative to the main scanning direction, themisalignment in image is corrected by controlling the positions wherethe image data are written, thereby eliminating the mechanicaladjustment. Therefore, the misalignment in image can be easily correctedin the line head for conducting multiple exposure.

Further, in the fourth image forming apparatus, since the delay controlis conducted for every block, the circuit structure can be simplified ascompared to the case in which the delay control is conducted for everylight emitting element. Since the delay control is conducted duringmultiple exposure, in case of forming an image formed by superposing aplurality of colors, the fourth image forming apparatus can form theimage without color registration error. Since the light emitting elementare controlled with signals formed by adding signal corresponding to thetiming shift between the light emitting element lines, the control forlight emitting elements can be simplified as compared to the case inwhich delay timings are set for all of the light emitting element lines.

In the fourth image forming apparatus of the present invention, sincestorage means for storing information of tilt of line head relative tothe main scanning direction is disposed in the apparatus body, even whenthe line head is out of order for any reason, the tilt information ofthe line head can be securely maintained. Since the storage means isdisposed in the cartridge to which the line head is mounted, the storagemeans can be replaced with a new storage means storing informationcorresponding to the tilt of new line heads automatically at the sametime as the replacement of the cartridge. Since the storage means isdisposed in the line head, after the replacement of the line head, thecontrol for light emitting elements can be conducted according to thetilt information of a new line head.

Further, in the fourth image forming apparatus of the present invention,since the light emitting elements are controlled by a driving circuitaccording to the active matrix method, the light emitting elements canbe maintained to keep emitting light by means of condensers andtransistors arranged around the light emitting elements. Therefore, thelight emitting elements remain to emit light even during thetransmission of image data from a shift resistor to the next shiftresistor, thereby exposing pixels with high luminance. In addition, thefourth image forming apparatus of the present invention is characterizedin that the amounts of light emitted from the light emitting elements recontrolled in the PWM method. Since the amount of exposure can bechanged by ON/OFF control of the light emitting elements, the circuitstructure can be simplified. Moreover, the amounts of light emitted fromthe light emitting elements are controlled in the intensify modulationmethod. Therefore, it is not required to control the ON/OFF of the lightemitting elements at a high speed. Even when the speed of response ofthe light emitting elements is slow, the amount of exposure can bechanged at a high speed. In addition, each of the light emittingelements comprises an organic EL. Therefore, the light emitting elementscan be easily formed on a glass substrate, thereby achieving lowerprice.

Further, the fourth image forming apparatus is adopted to an imageforming apparatus of a tandem type which comprises at least two imageforming stations each having an image carrier and further having acharging means, an exposure head, a developing means, and a transfermeans which are arranged around said image carrier and forms a colorimage by passing a transfer medium through the respective stations.Accordingly, in the image forming apparatus of a tandem type, themisalignment in image can be easily corrected.

A fifth image forming apparatus of the present invention is an imageforming apparatus comprising a line head in which a plurality of lineseach having a plurality of light emitting elements aligned in the mainscanning direction are arranged to have rows in the sub scanningdirection of an image carrier so that light emitting elements arearranged in a matrix in a plane, wherein pixels on said image carrierare exposed by the light emitting elements aligned in one line andexposed again by the light emitting elements aligned in the next lineafter the movement of said image carrier, and in the same manner, saidpixels are sequentially exposed by the light emitting elements onanother line after the movement of said image carrier so as to achievemultiple exposure of the pixels.

The fifth image forming apparatus being characterized by comprisingstorage means for storing information of tilt of the line head relativeto the main scanning direction, and control means for controlling lightemitting elements which protrude from the normal exposure line, amonglight emitting elements aligned in the sub scanning direction of saidline head, to emit smaller amount of light and controlling the imagedata to be supplied to light emitting elements in such a manner that theposition of image formation corresponding to the pixels on the imagecarrier is corrected from the tilt of the line head.

In the fifth image forming apparatus of the present invention, since thecontrol means for controlling light emitting elements which protrudefrom the normal exposure line to emit smaller amount of light andcontrolling the image data to be supplied to light emitting elements insuch a manner that the position of image formation corresponding to thepixels on the image carrier is corrected from the tilt of the line headis provided, outlines of pixels which are adjacent to each other in themain scanning direction can be clearly formed, thus preventing thedeterioration of printing quality.

A third image forming method of the present invention achieving theaforementioned object is an image forming method using a line head inwhich a plurality of light emitting element lines each having aplurality of light emitting elements aligned in the main scanningdirection are arranged in a matrix in a plane to have rows in the subscanning direction of an image carrier so that pixels on the imagecarrier are repeatedly exposed by light emitting elements on therespective lines to achieve the multiple exposure. The third imageforming method is characterized by comprising a step of storing theinformation of tilt of said line head relative to the main scanningdirection and a step of controlling the image data to be supplied tolight emitting elements by delaying the supply timing in such a mannerthat the position of image formation corresponding to the pixels on theimage carrier is corrected from the tilt of the line head.

According to the third image forming method of the present invention,even when the line heads are installed to the apparatus such that one ofthe line heads is tilted to the main scanning direction, an image can beformed by multiple exposure without need of mechanical adjustment of theline head and with the printing quality prevented from deteriorating.

A fourth image forming method is an image forming method using a linehead in which a plurality of light emitting element lines each having aplurality of light emitting elements aligned in the main scanningdirection are arranged in a matrix in a plane to have rows in the subscanning direction of an image carrier so that pixels on the imagecarrier are repeatedly exposed by light emitting elements on therespective lines to achieve the multiple exposure. The fourth imageforming method is characterized by comprising a step of storing theinformation of tilt of said line head relative to the main scanningdirection and a step of controlling light emitting elements whichprotrude from the normal exposure line to emit smaller amount of light,wherein the image data supplied to light emitting elements arecontrolled such that the position of image formation corresponding tothe pixels on the image carrier is corrected from the tilt of the linehead.

According to the fourth image forming method of the present invention,occurrence of misalignment in image due to the tilt of the line head canbe prevented with simple control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram partially showing an example of an imageforming apparatus according to the present invention;

FIG. 2 is a block diagram showing the entire structure of the exampleshown in FIG. 1;

FIG. 3 is an explanatory illustration showing an example of an imageforming apparatus according to another embodiment of the presentinvention;

FIG. 4 is a block diagram showing a control unit of the image formingapparatus shown in FIG. 3;

FIG. 5 is a block diagram showing a control unit of an image formingapparatus according to another embodiment of the present invention;

FIG. 6 is a circuit diagram showing a control circuit for light emittingelements which are driven in the active matrix method;

FIG. 7 is a table for explanation of an example of the relation betweenbit data and gradation data;

FIG. 8 is a block diagram of an example in which light emitting elementsare controlled by the pulse-width modulation (PWM) method;

FIGS. 9(a)-9(c) are characteristic graphs of an example in which thelight emitting elements are controlled by the PWM method;

FIG. 10 is a block diagram of an example in which the light emittingelements are controlled by the intensity modulation method;

FIG. 11 is a perspective view showing an example of organic EL arraysaccording to an embodiment of the present invention;

FIG. 12 is a sectional view showing the schematic structure of theorganic EL arrays;

FIG. 13 is an explanatory illustration showing a conventional example;

FIGS. 14(A), 14(B) are explanatory illustrations showing an example ofimage formation by the conventional example shown in FIG. 13;

FIG. 15 is a plan view showing a line head according to anotherembodiment of the present invention;

FIGS. 16(A), 16(B) are explanatory illustrations showing an example ofimage formation by the embodiment shown in FIG. 15;

FIG. 17 is an explanatory illustration showing an example of color imageformation after correction of color registration error;

FIGS. 18(A), 18(B) are explanatory illustrations showing a conventionalexample;

FIG. 19 is an explanatory illustration showing a conventional example;

FIG. 20 is an explanatory illustration showing a conventional example;

FIGS. 21(A), 21(B) are explanatory illustrations showing an example ofimage formation according to the present invention;

FIG. 22 is a block diagram showing an example of an image formingapparatus according to the present invention;

FIG. 23 is an explanatory illustration showing an example of imageformation according to the present invention;

FIG. 24 is an explanatory illustration showing an example of imageformation according to the present invention;

FIGS. 25(A), 25(B) are explanatory illustrations showing an example ofimage formation according to another embodiment of the presentinvention;

FIG. 26 is a block diagram showing a control unit corresponding to theexample shown in FIG. 25;

FIG. 27 is a block diagram showing control units according to an anotherembodiment of the present invention;

FIGS. 28(A), 28(B) are explanatory illustrations showing an example ofimage formation according to anther embodiment of the present invention;and

FIG. 29 is a front view showing the schematic structure of an imageforming apparatus of a tandem type in which the organic EL array head ofthe present invention is arranged.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in the below with reference tothe drawings.

FIG. 2 is a block diagram showing the schematic structure of an imageforming apparatus of the present invention. In FIG. 2, a host computer21 produces printing data and sends the printing data to a control unit22 of the image forming apparatus. The control unit 22 of the imageforming apparatus comprises a data processing means 23, storage means24-27, and light-emitting element line heads (optical heads) 28-31arranged corresponding to the aforementioned storage means 24-27. Thelight-emitting element line heads 28-31 correspond to four colors, i.e.yellow, magenta, cyan, and black, respectively, to form a color image ona photoreceptor. The storage means 24-27 store image data correspondingto light-emitting element line heads 28-31 for the respective colors.

The data processing means 23 carries out processes such as colorseparation, gradation treatment, bit-mapping of image data, andcorrection of color registration error. The data processing means 23outputs image data for each line to each storage means 24-27. Eachlight-emitting element line head 28-31 has a plurality of light emittingelement lines arranged therein and is structured to conduct multipleexposure in which light emitting elements on the respective lines emitlight to a same pixel. Therefore, each storage means 24-27 outputs imagedata for plural lines to each light-emitting element line head 28-31.

FIG. 1 is a block diagram partially showing the structure shown in FIG.2. FIG. 1 shows details of the light-emitting element (yellow) line head28 and the storage means 24 corresponding to the line head 28. In theexample shown in FIG. 1, the line head 28 has a line 28 a provided witha plurality of light-emitting elements 32. In this example, five lines28 a-28 e are arranged in the sub scanning direction X of an imagecarrier and each line has the same number of light-emitting elements.The storage means 24 comprise shift resistors 24 a-24 e to correspond tothe lines 28 a-28 e composed of the light-emitting elements,respectively. In FIG. 1, the direction of arrow X indicates the movingdirection (sub scanning direction) of a photoreceptor drum (imagecarrier) and the direction of arrow Y indicates the main scanningdirection.

Now, the operation of the block diagram shown in FIG. 1 will bedescribed. As the image data is inputted from the data processing means23 into the storage means 24, the shift resistor 24 a outputs image datato the light emitting elements in the first line 28 a so that the lightemitting elements work, whereby pixels on the image carrier are exposedto a predetermined amount of light. The image carrier is driven torotate in the direction of arrow X in such a manner that the pixelsexposed by the light emitting elements of the first line 28 a reach aposition corresponding to the light emitting elements arranged in thenext line 28 b. At the same time, the image data inputted in the shiftresistor 24 a are transmitted to the shift resistor 24 b.

The shift resistor 24 b outputs the image data to the light emittingelements of the line 28 b so that the light emitting elements work.Accordingly, the pixels previously exposed by the light emittingelements of the line 28 a are exposed again by the light emittingelements of the line 28 b with the equal amount of light. In thismanner, the image data is sequentially transmitted from the previousshift resistor to the next shift resistor while the image carrier ismoved in the direction of arrow X, whereby each same pixel is exposedagain and again by light emitting elements in different lines.Consequently, in the example of FIG. 1, the respective pixels areexposed to light of which amount is quintuple of that of a single lightemitting element, thereby quickly obtaining the amount of light requiredto expose each pixel. The number of the lines in which the lightemitting elements are aligned in the sub scanning direction can besuitably selected, that is, the number for multiplying the amount oflight for exposure to be obtained by a single light emitting element canbe suitably selected, as necessary.

In case of gradation control for neutral density is conducted by thestructure of FIG. 1, assuming that the predetermined luminance is 1,image data for luminance of 0.1 are inputted from the data processingdevice 23 to the shift resistor 24 a. As mentioned above, bytransmitting the image data sequentially to the shift resistors 24 a-24e to output the image data to the light emitting elements while movingthe image carrier, the luminance of each pixel becomes 0.1×5=0.5,providing a neutral density. In this manner, output for gradation whenexposing pixels can be obtained.

In the present invention, once the data processing means 23 of the imageforming apparatus produces data only for the front one line, the imagedata for the first line is stored in the storage means (shift resistor)and are transmitted among the storage means, whereby the operations ofall light emitting elements of the line head can be controlled. Sincethe data processing means is not required to produce data for all lightemitting elements of the line head, the structure of circuit can besimplified and the data processing can be conducted at high speed.

FIG. 3 shows the structure according to another embodiment of thepresent invention and is an explanatory diagram of spot positions 33 tobe formed on the image carrier. Hatched portions in FIG. 3 are spotpositions. Pixels at these positions are exposed to light. Positionsindicated by chain double-dashed lines are pixels not to be exposed tolight. “Pa” indicates a pixel pitch in the main scanning direction and“Pb” indicates a pixel pitch in the sub scanning direction. “S”indicates a pitch between spot positions (spot position pitch) in thesub scanning direction which is an integer multiple of the pixel pitch.In this example, the pitch is twice as the pixel pitch. As for the spotpositions 33 shown in FIG. 3, spots are formed in each of lines 33 a, 33c, 33 e, 33 g, and 33 i on the image carrier by light emitted from thelight emitting elements, thereby exposing pixels. In each of lines 33 b,33 d, 33 f, and 33 h, spots are not formed on the image carrier by lightemitted from the light emitting elements.

FIG. 4 is a block diagram corresponding to FIG. 3. Description will bemade as regard to the light-emitting element line heads 28X for yellowsimilarly to the description with respect to FIG. 1. The spot positions33 shown in FIG. 3 are formed by lines 28 f-28 n on which light emittingelements are aligned. Positions where no lines of light emittingelements are formed in the line head 28X of FIG. 4 correspond to thepositions where no pixels are exposed shown in FIG. 3. The storage means24 comprises a first group consisting of shift resistors 24 f-24 ncorresponding to the lines 28 f-28 n in which the light emittingelements are aligned, respectively. There is a second group consistingof shift resistors 24 g-24 m each of which is arranged between each pairof adjacent shift resistors among the aforementioned shift resistors 24f-24 n. The shift resistors 24 g-24 m of the second group operate onlyfor transmission of image data to the next shift resistor withoutoutputting the image data to light emitting elements.

In the example of FIG. 3 and FIG. 4, the operation for exposing pixelsat the spot positions 33 a in lines on the image carrier will bedescribed. The image data is outputted from the shift resistor 241 f tothe first line 28 f of light emitting elements, whereby pixels on theimage carrier are exposed. At the moment that the image carrier isdriven to rotate just for the pixel pitch Pb in the sub scanningdirection, the image data are transmitted from the shift resistor 24 fto the shift resistor 24 g. At this time, the shift resistor 24 g doesnot output the image data so that no pixel is exposed. At the momentthat the image carrier is further driven to rotate just for the pixelpitch in the sub scanning direction, the image data are transmitted fromthe shift resistor 24 g to the shift resistor 24 h. The shift resistor24 h outputs the image data to the light emitting elements of the line28 h. The light emitting elements on the line 28 h emit light to exposethe same pixels on the line in the spot positions 33 a.

In the same manner, the movement of the image carrier, the transmissionof the image data to the respective shift resistors, and the output ofthe image data to the light emitting elements are sequentiallyconducted, thereby achieving multiple exposure relative to same pixels.In this case, the gradation control for neutral density can be conductedon the basis of the data prepared by the data processing means 23.Though the lines of which pixels are exposed and the lines of whichpixels are not exposed are arranged alternately every line in theexample of FIG. 3, two lines of which pixels are not exposed may bearranged between the lines which pixels are exposed. That is, theexposure to pixels is conducted on every third line. In this case, inFIG. 4, two shift resistors which transmit image data without outputtingthe image data are connected vertically and the third shift resistorconducts the control of light emitting elements. In this manner, thepresent invention can provide a variety of image formation on the imagecarrier.

According to the present invention, even when the interval in the subscanning direction between the spot positions where the light emittingelements emit light to the image carrier is an integral multiple of thepixel pitch in the sub scanning direction, the multiple exposure of eachpixel can be achieved by arranging the respective shift resistors tocorrespond to the line with light emitting elements and line withoutlight emitting elements as shown in FIG. 3 and FIG. 4. In this case, thetiming for transmitting image data stored in a shift resistor to thenext shift resistor and the timing for making light emitting elements inthe line to emit light on the basis on the image data stored in theshift resistor are synchronized, thereby simplifying the circuitstructure and speeding up the operation. Though the spot position pitchin the sub scanning direction is twice as the pixel pitch in the exampleof FIG. 3, the spot position pitch may be other integral multiple of thepixel pitch. Therefore, the spot position pitch may be the same as thepixel pitch. In this case, the multiple number is 1.

FIG. 5 is a block diagram showing an image forming apparatus accordingto another embodiment of the present invention. The example shown inFIG. 5 is an apparatus in which light emitting elements are driven inthe active matrix method. In FIG. 5, “Z” indicates each single lightemitting part composed of a light emitting element and a driving circuitarranged according to the active matrix method. Five lines of lightemitting elements 28 p-28 t are arranged in a line head 28Y.Corresponding to the light emitting elements 28 p-28 t of the respectivelines, shift resistors 24 p-24 t are arranged. Connected to a dataprocessing means 23 is a line selector 34. Numeral 35 a designates asupply line of image data from the data processing means 23 to the shiftresistors, numeral 35 b designates a control line connecting the dataprocessing means 23 and the line selector 34, numerals 36 a-36 edesignate command lines for commanding action from the line selector 34to the respective shift resistors 24 p-24 t, numerals 37 a-37 edesignate scanning lines for supplying signals from the line selector 34to the light emitting elements of the respective lines, and numerals 38a-38 k designate signal lines for supplying operational signals from theshift resistors 24 p-24 t to individual light emitting elements (organicELs) in each line.

Description will now be made as regard to the operation of FIG. 5.According to a control signal supplied from the data processing means 23through the control line 35 b, the line selector 34 selects a scanningline 37 a and sends a signal to the line for light emitting elements 28p. In addition, the line selector 34 activates the shift resistor 24 paccording to the signal through the command line 36 a. The shiftresistor 24 p activates the signal lines 38 a-38 k to send outputsignals of image data to all of the light emitting elements 28 p in theline. The light emitting elements 28 p in the line emit lights to exposepixels. By shifting the scanning line 37 and the command line 36according to the signal from the line selector 34, the above actions arealso conducted for the light emitting elements 28 q, 28 r, 28 s, and 28t, whereby the light emitting elements in all lines are activated toemit light to expose the pixels.

Then, the image data in the shift resistor 24 s is transmitted to theshift resistor 24 t. In the same manner, the image data is sequentiallytransmitted from the shift resistor 24 r to the shift resistor 24 s,from the shift resistor 24 q to the shift resistor 24 r, and the shiftresistor 24 p to the shift resistor 24 q. To the shift resistor 24 p,image data is transmitted from the data processing means 23 through thesignal line 35 a. During this, the image carrier is moved for the pixelpitch. Since the light emitting elements at the light emitting parts Zremain to emit light because of the function of the active matrix, thelight emitting elements do not lights out even during the transmissionof image data between the shift resistors, thereby exposing pixels withhigh luminance. By repeating the outputting of image data from the shiftresistor 24 to the light emitting elements, the transmission of theimage data between the shift resistors, and the movement of the imagecarrier, the image data onto the image carrier can be consecutivelywritten.

FIG. 6 is a circuit diagram for operating the light emitting parts Zaccording to the active matrix. In FIG. 6, an organic EL is employed aseach light emitting element, “K” designates a cathode terminal thereofand “A” designates an anode terminal. The cathode terminal K isconnected to a power source which is not shown. “37 a” designates ascanning line which is connected to a gate Ga of a switching TFT (Tr1)“38 a” designates a signal line which is connected to a drain Da of theswitching TFT. “39” designates a power line and “Ca” designates astorage capacitor. A source Sb of a driving TFT (Tr2) of the organic ELis connected to the power line 39 and a drain Db is connected to theanode terminal A of the organic EL. In addition, a gate Gb of thedriving TFT is connected to a source Sa of the switching TFT.

Description will now be made as regard to the operation of the circuitshown in FIG. 6. As the signal line 38 a is energized in a state that avoltage of the power line 39 is applied to the source of the switchingTFT, the switching TFT is turned ON. Accordingly, the gate voltage ofthe driving TFT is lowered and the voltage of the power line 39 issupplied from the source of the driving TFT so that the driving TFTbecomes to the conducting state. As a result, the organic EL isactivated to emit light of a predetermined luminance. In addition, thestorage capacitor Ca is charged by the voltage of the power line 39.Even when the switching TFT is turned OFF, the driving TFT is still inthe conducting state according to the charge stored in the storagecapacitor Ca so that the organic EL remains to emit light. Therefore, byadopting the active matrix to the driving circuit for the light emittingelements, the operation of the organic EL is maintained to keep emittinglight even when the switching TFT is turned OFF for transmitting theimage data between the shift resistors, thereby exposing pixels withhigh luminance.

In the present invention, by controlling the light emitting elements inthe pulse-width modulation (PWM) method, the control of the amount ofemitting light is conducted. By the control according to the PWM method,the gradation control for the light emitting elements can be achieved.In the present invention, gradation data is formed by an 8-bit gradationdata memory.

FIG. 7 is a table for explanation of an example of the relation betweenbit data and gradation data stored in gradation data memories. In theexample of FIG. 7, the bit datum No. 1 is a gradation datum 0 (no lightemission), the bit datum No. 8 is a datum of the most condensed density,and the bit data No. 2-No. 7 are data of neutral densities therebetween.

FIG. 8 is a block diagram of an example for conducting PWM control. InFIG. 8, a PWM control unit 70 is provided with gradation data memories71 a, 71 b . . . composed of shift resistors or the like, a counter 72,comparators 73 a, 73 b. . . , and light emitting parts Za, Zb . . . .Supplied to the gradation data memories 71 a, 71 b . . . is a gradationdata signal 74 from, for example, the data processing means 23 shown inFIG. 5. The gradation data memories 71 a, 71 b . . . are 8-bit memoriesas shown in FIG. 7. The counter 72 counts reference clock signal 75. Thebit number of the counter 72 is eight bit which is the same as that ofthe gradation data memories 71 a, 71 b . . . so that the count repeats0→the maximum (255)→0→the maximum. The comparators 73 a, 73 b comparethe signal of the counter 72 to the gradation data stored in thegradation data memories 71 a, 71 b . . . . When the gradation data>thecounter value, the switching TFT is turned ON as shown in FIG. 6. Whenthe gradation data≦the counter value, the switching TFT is turned OFF.

FIGS. 9(a)-9(c) are characteristic graphs showing a concrete example ofthe PWM control shown in the block diagram of FIG. 8. FIG. 9(a) showsthe output Da of the counter 72 which repeats 0→the maximum (255)→0→themaximum→0 . . . as described in the above. FIG. 9(b) shows the waveformDb of the signal outputted from the comparator, i.e. the operatingcharacteristics of the switching TFT, when the gradation datum is thebit datum No. 7 (128 gradation level). In this case, the switching TFTis turned ON when the output of the counter is in a range of from 0 to127, and the switching TFT is turned OFF when the output of the counteris in a range of from 128 to 255.

FIG. 9(c) shows the waveform Dc of the signal outputted from thecomparator, i.e. the operating characteristics of the switching TFT,when the gradation datum is the bit datum No. 6 (64 gradation level). Inthis case, the switching TFT is turned ON when the output of the counteris in a range from 0 to 63, and the switching TFT is turned OFF when theoutput of the counter is 64 and 255. In case of FIG. 9(b), the pulsewidth of the waveform Db is Wa. In case of FIG. 9(c), the pulse width ofthe waveform Dc is Wb. That is, according to the size of the gradationdata, the time period for which the switching TFT is turned ON ischanged, thereby changing the amount of light emitted from the lightemitting elements. Since the amount of exposure to the image carrier canbe changed by ON/OFF of the light emitting elements according to theON/OFF control of the switching TFT, the circuit structure can besimplified.

FIG. 10 is a block diagram showing another structure according to thepresent invention. The same parts as used in FIG. 8 are marked with thesame numerals or marks, so the detail description about such parts willbe omitted. The example shown in FIG. 10 controls the switching TFT withvoltages or currents corresponding to the sizes of the gradation data.Such control as shown in FIG. 10 is called “Intensity Modulation” in thepresent invention. In an intensity modulation control unit 80 shown inFIG. 10, D/A converters 81 a, 81 b . . . are connected to the gradationdata memories 71 a, 71 b . . . , respectively. The D/A converters 81 a,81 b . . . form voltage values or current values of analog correspondingto the sizes of the gradation data stored in the gradation data memories71 a, 71 b . . . and output the voltage values or current values to theswitching TFTs, respectively.

In the example of FIG. 10, the amount of light emitted from the lightemitting elements is changed by changing the bias of the switching TFTcorresponding to the gradation data. Therefore, it is not required tocontrol the ON/OFF of the light emitting elements at a high speed. Evenwhen the speed of response of the light emitting elements is slow, theamount of exposure to the image carrier can be changed at a high speed.Light emitting parts Za, Zb . . . are driven in the active matrix methodshown in FIG. 6. Supplied to the light emitting parts Za, Zb . . . are aselect signal through a scanning line 37 a and control signals throughemission control data lines 38 a, 38 b. . . .

In the present invention, organic EL (organic electroluminescenceelement) arrays are employed in lines of light emitting elements formultiple exposure. FIG. 11 is a perspective view showing an example ofthe organic EL array to be employed in the image forming apparatus ofthe present invention. In FIG. 11, an organic EL array 12 is mounted ona rectangular substrate 1 made of glass or the like. The organic ELs areconnected to a driving circuit 11 for controlling the emission. Therectangular substrate 1 is provided with positioning pins 13 and throughholes 14 for installation formed on both sides thereof. Numeral 16designates a protective cover for covering the driving circuit 11 andthe organic EL array 12. A condensing rod lens array 15 as magnifyingoptical system is fixed on the side of the image carrier. Because of thecondensing function of the condensing rod lens array 15, light-emittingparts of the organic EL array 12 are condensed to form an image on aphotosensitive surface of the image carrier.

FIG. 12 is a vertical sectional front view showing an example of anorganic EL array head 10. In FIG. 12, a reflection layer 2 composed ofdielectric multi-layered film is formed on the substrate 1, made ofglass or a resin film, by the spattering method. The reflection layer 2composed of a dielectric multi-layered film may be formed of, forexample, a pair of layers made of SiO₂ and TiO₂. The reflective layer 2formed of such a dielectric multi-layered film has reflectance of 0.99or more.

An anode 3 is formed on the reflection layer 2 by the spattering method.The anode 3 is made of a light-transmitting and conductive material. Asan example of the material having such characteristics, ITO (indium tinoxide) having large working function may be used. Then, a holetransportation layer 4 is formed on the anode 3 by the inkjet method.After forming the hole transportation layer 4, ink composition isdischarged into the hole (not shown) from a head of an ink-jet printingdevice, thereby achieving the patterning application on the emittinglayer of the pixel. After the application, the solvent is removed andthe applied ink composition is treated by heat, thereby forming alight-emitting layer 5.

The organic EL layer composed of the hole transportation layer 4 and theemitting layer 5 may be formed by other known method such as a spincoating method, a dipping method, and other liquid phase depositionmethod instead of applying ink compositions by inkjet method as theabove. The material of the hole transportation layer 4 and the emittinglayer 5 may be known EL materials listed in Japanese Patent UnexaminedPublication No. H10-12377 and Japanese Patent Unexamined Publication No.2000-323276, so description about details will be omitted. Then, acathode 6 is formed by vapor deposition method. As the material of thecathode 6, for example, Al may be employed.

The organic EL array head 10 has thin layer portions 6 a-6 c formed atthe cathodes 6 having a U-like section corresponding to light emittingparts 10 x-10 z. The thin layer portions 6 a-6 c are formed to have sucha thickness in holes of a wall 9 as to allow light transmission. At thelight emitting parts 10 x-10 z, semi-transparent reflection layers(dielectric mirrors) 7 composed of a plurality of dielectricmulti-layered films are formed on the bottoms of the cathodes 6 by thespattering method. The semi-transparent reflection layers 7 a-7 ccomposed of dielectric multi-layered films may be formed of, forexample, three pairs of layers made of SiO₂ and TiO₂. Thesemi-transparent reflection layers 7 formed of such dielectricmulti-layered films according to the present invention has reflectanceof about 0.9. In the embodiment of FIG. 12, as mentioned above, the thinlayer portions 6 a-6 c are formed at the cathodes 6, thereby allowinglight transmittance. Accordingly, even when the organic EL layercomposed of the hole transportation layer 4 and the emitting layer 5 isformed by a liquid phase deposition method such as the inkjet method, itis free from the possible problem that the reflectance is reduced due tothe smoothness of contact portion between the EL layer and the cathode.In the present invention, the organic EL array head having theaforementioned structure can be used as an exposure head of an imageforming apparatus, for example, capable of forming a color image byusing electrophotographic technique.

By the way, a line head in which light emitting elements are aligned ina plurality of rows has a possible problem that the mounted position ofthe head to an apparatus is easily shifted due to a problem caused inthe manufacturing process. FIG. 13 is an explanatory illustrationshowing an example where the line head is installed to the apparatus ina state that the mounted position of the line head is shifted. In FIG.13, numeral 28 designates a line head, 61 designates a pair of fixingrollers (fixing device), “P” designates a paper sheet, “W” designatesthe feeding direction of the paper sheet P. “Ta” designates the normalmounted position of the line head 28. In the example shown in FIG. 13,the installed line head is shifted so that the position of one edge ofthe line head protrudes from the normal mounted position Ta. Therefore,there is an installation error Tb. As the line head is installed to theapparatus body in a state that the mounted position of the line head isshifted from the normal mounted position, the image exposure position isalso shifted. Consequently, it is impossible to form a part of image,thus providing significantly poor image quality. FIGS. 14(A), 14(B) areexplanatory illustrations showing an example where a part of image isnot formed as mentioned above.

FIG. 14(A) shows an example of image formation when the line head isinstalled at the normal position. In this case, image is formed on apaper sheet P including the front row Ea of the image from the imageformation reference position V. On the other hand, FIG. 14(B) shows acase where the line head 28 is installed at a position shifted from thenormal position as shown in FIG. 13, the image is formed on a papersheet P such that the front row Ea is shifted for two rows. In a colorprinter having respective exposure units for four colors, the respectiveexposure units are independently installed and the alignment amongimages formed by the respective exposure units is difficult. Unless eachexposure unit is installed to a position precisely parallel to the otherexposure units, toner images of respective colors are never neatlysuperposed on each other, thus providing poor image quality. To preventthe misalignment among the respective exposure units as shown in FIG.13, it is required to improve the accuracy in alignment of the entireprinter, causing a problem of increasing the cost. In addition, it isnecessary to conduct alignment of the respective exposure units withtest printing so that there is a problem that the alignment takes timeand makes the operation complex.

Even when the mounted position of the line head is shifted from thenormal position as mentioned above, an embodiment of the presentinvention can maintain the image quality without complex alignment ofthe mounted positions of line heads. FIG. 15 is a plan view showing aline head of the present invention. In FIG. 15, the line head 28 has alarge number of light emitting elements 32 which are aligned in aplurality of rows and in a plurality of lines.

In the line head 28 shown in FIG. 15, light emitting elements from oneend to rows Ra in the longitudinal direction are light emitting elementsfor normal exposure. Light emitting elements of two rows from the otherend are light emitting elements which are preliminarily arranged foradjusting the position of image (resist light emitting elements). Thoughthe number of rows of resist light emitting elements is two in theembodiment shown in FIG. 15, the number is not limited to two and may besuitably set.

FIGS. 16(A), 16(B) are explanatory illustrations showing an example ofimage formation by the line head shown in FIG. 15. FIG. 16(A) shows anormal image, “Rc” indicates light emitting elements in two rows on thefront side, and “Rb” indicates resist light emitting elements in tworows on the rear side. Hatched circles indicate pixels to be formed andopen circles indicate pixels to be not formed. “V” designates the imageforming reference position and “Ea” is the first row of the portionwhere image is formed. FIG. 16(B) shows an image after corrected. Inthis case, image is not formed by the light emitting elements Rc in thetwo rows of the front side while image is formed by the light emittingelements (resist light emitting elements) Rb in the two row of the rearside. Therefore, even when the line head is installed to the apparatusat a position shifted from the normal position, the image quality can bemaintained. In FIGS. 16(A), 16(B), the light emitting element lines arearranged in a plurality of lines, thereby conducting the multipleexposure.

Now, the control example for correcting the misalignment as shown inFIGS. 16(A), 16(B) will be described with reference to FIG. 5. As theline head 28 is installed to the apparatus at a position shifted fromthe normal position as shown in FIG. 13, information of misalignment isobtained by a sensor which is not shown in drawing. The information ofmisalignment is stored in a suitable storing means, for example, amemory in the data processing means 23 in FIG. 5. When signals are sentfrom the data processing means 23 to the shift resistors 24 p-24 t foractuating the respective light emitting elements, blank data areoutputted through the signal lines 38 a, 38 b for all of the lightemitting element lines 28 p-28 t so that the light emitting elements intwo rows on the front side do not form image.

FIG. 17 is an explanatory illustration showing an example of color imageformation according to the present invention. In FIG. 17, (K) shows ablack image, (C) shows a cyan image, (M) shows a magenta image, and (Y)shows an yellow image. In this example, the position of the line headfor magenta is shifted. The correction as described in FIGS. 16(A),16(B) is conducted relative to the line head for magenta. Accordingly,when four unicolor images are superposed on each other, suitable imageformation can be conducted because the effect of misalignment in mountedpositions of the line head is corrected.

The line head in which light emitting elements are aligned in aplurality of rows has a possible problem that the mounted position ofthe head to an apparatus is easily tilted relative to the main scanningdirection due to a problem caused in the manufacturing process. FIGS.18(A), 18(B) are explanatory illustrations showing an example where theline head is installed to the apparatus in a tilted state. FIG. 18(A)shows a line head 28. The line head 28 has light emitting element lines28 a-28 c each provided with a plurality of light-emitting elements Z inthe main scanning direction. Here, “Ya” indicates the main scanningdirection and “W” indicates the paper feeding direction (sub scanningdirection). In the example shown in FIG. 18(A), the line head 28 isinstalled to the apparatus in a state tilted relative to the mainscanning direction Ya. For example, a light emitting element Zx arrangedat one end of the line head 28 is out of position of the main scanningdirection Ya. In this case, the line of formed image 33 is not parallelto the line of the main scanning direction Ya as shown in FIG. 18(B). Asthe mounted position of the line head is tilted relative to the mainscanning direction, the exposure position for image is shifted so thatthe line of pixels which should be formed parallel to the main scanningdirection under normal conditions has an angle relative to the mainscanning direction, thus causing a problem of deteriorating printingquality.

FIG. 19 is an explanatory illustration showing an example of imageformation in case where the line head is tilted relative to the mainscanning direction as shown in FIGS. 18(A), 18(B). In FIG. 19, numerals33 a-33 i designate pixel lines and Ha-Hn designate pixel rows. In thiscase, the exposure line width of the pixel rows Ha-Hn is increased. Thatis, pixels 33 x to be formed on the central line La under normalconditions become pixels 33 y shifted to have a central line Lb, thusincreasing the exposure line width in the main scanning direction Ya.Accordingly, there is a problem that the outlines of pixels which areadjacent to each other in the main scanning direction are superposed oneach other so that the image must be fuzzy and the image quality ispoor. In a color printer having respective exposure units for fourcolors, the respective exposure units (line heads) are independentlyinstalled and the alignment among images formed by the respectiveexposure units is difficult. As one of the exposure units is installedto the apparatus in s state tilted relative to the main scanningdirection, the respective unicolor toner images can not be neatlysuperposed on each other, thus deteriorating the image quality.

FIG. 20 is an explanatory illustration showing an example of imageformation in case where line heads are installed to the apparatus bodyof a color printer in a state tilted relative to the main scanningdirection. The line heads 28 x shown in FIG. 20 are a line head forblack (K), a line head for cyan (C), a line head for magenta (M), and aline head for yellow (Y). Each line head has a plurality of lightemitting element lines. In the example shown in FIG. 20, the line headfor magenta (M) is installed to the apparatus in a state tilted relativeto the main scanning direction Ya. First, a pixel line 33 p is formed ona paper sheet by the line head for yellow (Y). Then, the paper sheet isfed in the direction W and a pixel line 33 q is formed to be superposedon the pixel line 33 p on the paper sheet by the line head for magenta(M).

Since the line head for magenta (M) is tilted relative to the mainscanning direction Ya, however, the pixel line 33 q is not neatlysuperposed on the pixel line 33 p. Then, the paper sheet is fed in thedirection W and a pixel line 33 r is formed to be superposed on thepixel line 33 p by the line head for cyan (C). The paper sheet isfurther fed in the direction W and a pixel line 33 s is formed to besuperposed on the pixel lines 33 p, 33 r by the line head for black (K).Therefore, in the example of FIG. 20, the pixel line 33 q formed by theline head for magenta (M) is not parallel to the line in the mainscanning direction, thus causing color registration error relative toother colors and deteriorating the printing quality.

To prevent any one of the line heads for exposure of plural colors frombeing tilted like the above example, it is required to improve theaccuracy in alignment of the entire printer, causing a problem ofincreasing the cost. In addition, it is necessary to conduct alignmentof the respective line heads with test printing so that there is aproblem that the alignment takes time and makes the operation complex.

FIGS. 21(A), 21(B) are explanatory illustrations showing the structureof another embodiment according to the present invention. FIG. 21(A)shows a line head. The line head 28 has light emitting element lines 28a-28 c. In the example shown in FIG. 21(A), the line head 28 isinstalled to the apparatus in a state tilted relative to the mainscanning direction Ya. FIG. 21(B) shows an image after the misalignmentdue to the tilt of the line head is corrected. In the line head 28, aplurality of light emitting element lines each having a plurality oflight emitting elements are arranged to have rows in the sub scanningdirection of the image carrier (the paper feeding direction w) so thatlight emitting elements are arranged in a matrix in a plane. Thisembodiment of the present invention is characterized by shifting thetiming of operation in a direction of the light emitting element rowsRa-Rn during the light emitting elements on the light emitting elementlines 28 a-28 c are operated.

That is, as taken from the paper feeding direction W, the operationtiming for the pixel row Ra, of which the front light emitting elementZx protrudes from the main scanning direction Ya, is delayed for apredetermined period of time. It should be noted that, in the presentinvention, the array of light emitting elements in the main scanningdirection is referred to as a light emitting element line and the arrayof light emitting elements in the paper feeding direction (sub scanningdirection) is referred to as a light emitting element row. The pixel rowRn, of which the front light emitting element Zy does not protrude fromthe main scanning direction Ya, is set not to delay the operationtiming. In the example shown in FIG. 21(A), the light emitting elementlines 28 a-28 c are tilted linearly relative to the main scanningdirection Ya. The times for delaying the operation timing of therespective pixel rows are set to increase gradually from Rn to Ra. Bythis control, the pixel line 33 is formed to be parallel to the line ofthe main scanning direction Ya as shown in FIG. 21(B), thereby cancelingthe misalignment in image and preventing the deterioration of printingquality.

FIG. 22 is a block diagram showing the structure of a control unit in animage forming apparatus of the present invention. In FIG. 22, numeral 22designates a control unit for an engine controller. An apparatus-sidecontroller 21 inputs image data to the first shift resistor 24 a in thecontrol unit 22. The first shift resistor 24 a outputs the image data torespective light emitting lines of the light emitting element line head28. That is, the first shift resistor 24 a functions as an image datasupplying means for supplying image data to the respective lightemitting elements. In the embodiment of the present invention, therespective light emitting element rows are operated with the respectivedelayed timing as described above with reference to FIG. 21(A).Therefore, the output signal of the first shift resistor 24 a is delayedfor a predetermined period of time via a delay circuit 40. Delay signalsoutputted from the delay circuit 40 through signal lines 38 a-38 n areformed according to the tilt information previously stored in a memory50. The memory 50 stores the tilt information of the line head.

The tilt information of the line head is obtained from the memory 50 andthe delay circuit 40 sets the level of delay time for each lightemitting element row according to the degree of tilt of the lightemitting element lines relative to the main scanning direction. Theoutput signal from the delay circuit 40 is given to the light emittingelement line head 28 through the second shift resistor 24 b. The secondshift resistor 24 b outputs signals through signal lines 38 a-38 n,thereby sequentially operating the light emitting elements of the lightemitting element lines 28 a-28 c of FIG. 21(A).

As mentioned above, in the control unit 22, the image data supplied fromthe delay circuit 40 to the light emitting elements are controlled to bedelayed according to the tilt information of the line head stored in thememory 50 in such a manner as to correct image positions of pixels on animage carrier from tilting due to the tilt of the line head. The delaycontrol for the timing of supplying image data to the light emittingelements can be carried out, for example, by providing a CPU, which isnot shown in drawing, to the delay circuit. The aforementioned memory 50may be arranged in the engine controller separately from the line head.In this case, even when the line head is out of order for any reason,the tilt information of the line head can be securely maintained. Thememory 50 may be formed integrally with the line head 28. In this case,since tilt information of new line head is stored in a memory (storagemeans) during the replacement of the line head, the control for lightemitting element rows can be conducted according to the tiltinformation. The storage means may be formed in a cartridge includingexposure units as will be described later. In this case, the storagemeans can be replaced with a new storage means storing informationcorresponding to the tilt of new line heads at the same time as thereplacement of the cartridge.

Instead of supplying delay signals to the light emitting element linesin FIG. 22, the following simple control may be conducted. That is,delay control signal for correcting the tilt of the line head isinputted into the front light emitting element line (28 a) as taken fromthe paper feeding direction in FIG. 21(A). On the other hand, the lightemitting element lines (28 b, 28 c) including and after the second lightemitting element line are controlled with signals formed by addingsignal corresponding to the timing shift from the former light emittingelement line to the aforementioned delay control signal for the frontlight emitting element line. According to this control method, thecontrol for light emitting elements can be simplified as compared to thecase in which delay timings are set for all of the light emittingelement lines.

FIG. 24 is an explanatory illustration corresponding to FIG. 20. Thisexample is an example of image formation with four colors. In theexample shown in FIG. 24, a line head for magenta (M) is installed tothe apparatus in a state tilted relative to the main scanning directionYa. First, a pixel line 33 p is formed on a paper sheet by a line headfor yellow (Y). Then, the paper sheet is fed in the direction W and apixel line 33 q is formed to be superposed on the pixel line 33 p on thepaper sheet by the line head for magenta (M) In this case, as describedwith regard to FIG. 22, the misalignment in image due to the tilt of theline head for magenta (M) is corrected. Therefore, misalignment betweenthe pixel line 33 p and the pixel line 33 q does not occur. Then, thepaper sheet is fed in the direction W and a pixel line 33 r is formed tobe superposed on the pixel lines 33 p, 33 q by a line head for cyan (C).The paper sheet is further fed in the direction W and a pixel line 33 sis formed to be superposed on the pixel lines 33 p, 33 q, 33 r by a linehead for black (K).

Though the line head for magenta (M) is installed to the apparatus in astate tilted relative to the main scanning direction Ya, the pixel line33 q formed by the line head for magenta (M) is parallel to the line ofthe main scanning direction Ya in the example of FIG. 24. Therefore, themisalignment from the other colors does not occur, thereby preventingthe deterioration of printing quality.

FIGS. 25(A), 25(B) are explanatory illustrations showing anotherembodiment according to the present invention. FIG. 25(A) shows a linehead 28 and FIG. 25(B) shows a pixel line 33 of image formed by the linehead 28 after correcting the tilt of the line head 28. In the exampleshown in FIG. 24(A), the line head 28 is installed to the apparatus in astate tilted relative to the main scanning direction Ya. In thisexample, blocks Rm-Rz including several light emitting element rows areformed.

In the example of FIG. 25(A), delay timing for light emitting elementsis set for every block. In this case, the circuit structure of thecontrol unit can be simplified as compared to the case in which delaytimings are set for all of the light emitting element lines like thecase shown in FIG. 21(A). As shown in FIG. 25(B), slight differences arecreated in the pixel line which is formed after the tilt of the linehead is corrected. However, these differences are slight to cause notrouble in practice. Therefore, also in the example of FIG. 25(A), thedeterioration of printing quality due to the tilt of line head can bereduced.

FIG. 26 is a block diagram of a control unit corresponding to FIG.25(A). The same parts as those used in FIG. 22 are marked with the samenumerals so that detail description about such parts will be omitted. InFIG. 26, the delay circuit 40 is provided with control signal formingportions 51 a-51 n corresponding to the blocks of the light emittingelement rows, respectively. Stored in the memory 50 is tilt informationof the line head 28. For example, a signal from the control signalforming portion 51 a of the delay circuit 40 is given to the lightemitting element line head 28 through the second shift resistor 24 b soas to operate light emitting elements. Signals passing through thesignal lines 38 a-38 c are the same. In the example of FIG. 25(A), thelight emitting elements in the light emitting element rows in the blockRm are operated with the same delay timing. Since delay control isconducted to the image data to be supplied to the light emittingelements for every block in the example of FIG. 26 as mentioned above,the circuit structure of this example can be simplified as compared tothe case in which delay control is conducted for every light emittingelement like the example of FIG. 22.

FIG. 27 is a block diagram showing another embodiment of the presentinvention. In the example of FIG. 27, control units are provided tocorrespond to line heads for four colors, i.e. black (K), cyan (C),magenta (M), yellow (Y), respectively. In FIG. 27, numeral 21 adesignates an apparatus-side controller corresponding to the respectiveline heads for four colors. Numeral 22 a designates a control unitcorresponding to the control units for the line head for black (K), 22 bdesignates a control unit corresponding to the line head for cyan (C),22 c designates a control unit corresponding to the line head formagenta (M), and 22 d designates a control unit corresponding to theline head for yellow (Y). In addition, memories 50 a-50 d are providedto correspond to the control units 22 a-22 d, respectively. According tothe structure shown in FIG. 27, in a color printer having respectiveline heads for four colors as shown in FIG. 24, the control units can beadopted to control the light emitting element rows when one of the lineheads is tilted. In addition, each of the delay circuits 40 of thecontrol units 22 a-22 d may be structured to set a delay timing forevery block like the example of FIG. 26.

FIGS. 28(A), 28(B) are explanatory illustrations showing anotherembodiment according to the present invention. In the example shown inFIGS. 28(A), 28(B), the effect of the tilt of the line head 28 iscorrected by changing the amounts of light of the light emitting elementlines 28 a-28 c. That is, since the line head 28 is tilted relative tothe main scanning direction Ya, the light emitting elements are shiftedin the width direction among the light emitting element rows Ra-Rn. Forexample, in the light emitting element rows Ra, the light emittingelement Zq of the light emitting element line 28 b is defined as thereference. In this case, on the basis of the center line CL of the lightemitting element Zq, the light emitting element Zp of the light emittingelement line 28 a protrudes to the left in the drawing. On the otherhand, the light emitting element Zr of the light emitting element line28 c protrudes to the right in the drawing on the basis of the centerline CL of the light emitting element Zq.

Accordingly, the light emitting element Zp and the light emittingelement Zr protrude from the width of the exposure line. Therefore, theoutlines of pixels which are adjacent to each other in the main scanningdirection are superposed on each other, thus deteriorating the imagequality as described with reference to FIG. 19. In the embodiments asdescribed in the above to cope with this problem, the amounts of lightof the light emitting elements are equal and the delay circuit(s) isused to delay the operation timings of light emitting elements for everylight emitting element row or for every block including a plurality oflight emitting rows.

The example of FIGS. 28(A), 28(B) adjusts the amounts of light of lightemitting elements in addition to the use of the delay circuit(s). Thatis, on the basis of the amount of light of each light emitting elementon the light emitting element line 28 b, the amount of light of eachlight emitting element on the light emitting element lines 28 a, 28 cabove and below the light emitting element line 28 b is reduced.Therefore, image to be formed by the light emitting elements protrudingin the width direction of the exposure line is prevented to be formed,thereby preventing the outlines of pixels which are adjacent to eachother in the main scanning direction from being superposed on each otheras shown in FIG. 23 and therefore maintaining the printing quality well.FIG. 23 is an explanatory illustration corresponding to FIG. 19, butaccording to the embodiment of the present invention. The partscorresponding to the parts used in FIG. 19 are marked with the samenumerals. FIG. 23 shows an example in which the outlines of pixels whichare adjacent to each other in the main scanning direction are notsuperposed on each other.

FIG. 29 is a front view showing an example of an image forming apparatusemploying the organic EL array head described with reference to FIG. 12.The image forming apparatus is of a tandem type in which four similarorganic EL array exposure heads 1K, 1C, 1M and 1Y are disposed at therespective exposure positions of four similar photoreceptor drums (imagecarriers) 41K, 41C, 41M and 41Y corresponding thereto. As shown in FIG.29, the image forming apparatus has a driving roller 51, a driven roller52, and a tension roller 53 and has an intermediate transfer belt 50.The intermediate transfer belt 50 is laid around the driving roller 51and the driven roller 52 with a certain tension applied by the tensionroller 53 and is driven to circulate in the direction of the arrowsshown in FIG. 29 (counterclockwise direction) by the driving roller 51.Four photoreceptor drums 41K, 41C, 41M and 41Y are disposed atpredetermined distance along the intermediate transfer belt 50. Eachphotoreceptor drum has a photosensitive layer on the outer peripheralsurface thereof to serve as an image carrier.

Suffixes “K”, “C”, “M”, and “Y” added to reference numerals indicateblack, cyan, magenta, and yellow, respectively. That is, thephotoreceptor drums designated by reference numerals with such suffixesare photoreceptor drums for black, cyan, magenta, and yellow,respectively. The same is true for other members. The photoreceptordrums 41K, 41C, 41M and 41Y are driven to rotate in the direction ofarrows shown in FIG. 29 (clockwise direction) synchronously with thedriving of the intermediate transfer belt 50. Arranged around eachphotoreceptor drum 41 (K, C, M, Y) are a charging means (corona charger)42 (K, C, M, Y) for uniformly charging the outer peripheral surface ofthe photoreceptor drum 41 (K, C, M, Y), an organic EL array exposurehead 1 (K, C, M, Y) having the aforementioned structure of the presentinvention for sequentially line-scanning the outer peripheral surface ofthe photoreceptor drum 41 (K, C, M, Y), which has been uniformly chargedby the charging means 42 (K, C, M, Y), synchronously with the rotationof the photoreceptor 41 (K, C, M, Y).

Also arranged around each photoreceptor drum 41 (K, C, M, Y) are adeveloping device 44 (K, C, M, Y) for applying toner as a developer toan electrostatic latent image formed by the organic EL array exposurehead 1 (K, C, M, Y) so as to form a visible image (toner image), aprimary transfer roller 45 (K, C, M, Y) serving as transfer means forsequentially transferring the toner image developed by the developingdevice 44 (K, C, M, Y) onto the intermediate transfer belt 50 as aprimary transfer target, and a cleaning device 46 (K, C, M, Y) ascleaning means for removing the toner remaining on the surface of thephotoreceptor drum 41 (K, C, M, Y) after the transfer of the tonerimage. Each organic EL array exposure head 1 (K, C, M, Y) is installedin such a manner that the array direction of the organic EL arrayexposure head 1 (K, C, M, Y) is parallel to the bus-bar of thephotoreceptor drum 41 (K, C, M, Y). The emission energy peak wavelengthof each organic EL array exposure head 1 (K, C, M, Y) and thesensitivity peak wavelength of the photoreceptor drum 41 (K, C, M, Y)are set to be approximately coincident with each other.

The developing device 44 (K, C, M, Y) uses anon-magneticsingle-component toner as a developer, for example. The single-componentdeveloper is conveyed to a development roller through a supply roller,for example, and the thickness of the developer layer adhering to thedevelopment roller surface is regulated with a regulating blade. Thedevelopment roller is brought into contact with or pressed against thephotoreceptor drum 41 (K, C, M, Y) to allow the developer to adhere tothe surface of the photoreceptor drum 41 (K, C, M, Y) according to theelectric potential level thereof, thereby developing the electrostaticlatent image into a toner image. Toner images of black, cyan, magentaand yellow formed by unicolor toner image forming stations for the fourcolors are sequentially primarily transferred onto the intermediatetransfer belt 50 by a primary transfer bias voltage applied to therespective primary transfer rollers 45 (K, C, M, and Y), andsequentially superimposed on each other on the intermediate transferbelt 50 to form a full-color toner image, which is then secondarilytransferred onto a recording medium “P” such as a paper at a secondarytransfer roller 66. The transferred full-color toner image is fixed onthe recording medium “P” by passing between a pair of fixing rollers 61as a fixing device. Then, the recording medium “P” is discharged througha pair of sheet delivery rollers 62 onto an outfeed tray 68 formed onthe top of the apparatus body.

In FIG. 29, reference numeral 63 designates a sheet cassette in which astack of a large number of recording media P is held, 64 designates apickup roller for feeding the recording medium P from the sheet cassette63 one by one, 65 designates a pair of gate rollers for regulating thetiming at which each recording medium P is supplied to the secondarytransfer portion at a secondary transfer roller 66, 66 designates thesecondary transfer roller as a secondary transfer means for forming thesecondary transfer portion together with the intermediate transfer belt50, 67 designates a cleaning blade as cleaning means for removing thetoner remaining on the surface of the intermediate transfer belt 50after the secondary transfer. As mentioned above, since the organic ELarray shown in FIG. 12 is used as the writing means in the image formingapparatus shown in FIG. 29, the apparatus can be manufactured to havesmaller size than a case using laser scanning optical system as thewriting means.

Though the image forming apparatus and the image forming method of thepresent invention have been described with reference to the embodimentsdisclosed herein, the present invention is not limited thereto andvarious modifications may be made therein.

Industrial Applicability

The present invention as described in the above can provide an imageforming apparatus and an image forming method, which are directed tosimplify the circuit structure and to speed up the light emittingcontrol during the exposure of pixels on an image carrier in multipleexposure manner capable of outputting gradation, at low cost.

1. An image forming apparatus in which a plurality of lines each havinga plurality of light emitting elements are arranged to have rows in thesub scanning direction of an image carrier so that light emittingelements are arranged in a matrix in a plane, wherein pixels on saidimage carrier are exposed by the light emitting elements aligned in oneline and exposed again by the light emitting elements aligned in thenext line after the movement of said image carrier, and in the samemanner, said pixels are sequentially exposed by the light emittingelements on another line after the movement of said image carrier so asto achieve multiple exposure of the pixels, said image forming apparatusbeing characterized by comprising control means for controlling saidlight emitting elements, which are disposed on the respective lines forexposing a same pixel, to emit the same amount of light, wherein thepixels can be exposed according to gradation output formed by saidcontrol means.
 2. An image forming apparatus as claimed in claim 1,being characterized by comprising storage means for storing image dataformed by said control means and outputting said image data to saidlight emitting elements, wherein said storage means is composed of meanswhich are arranged to correspond to the lines of the light emittingelements, respectively and are designed to transport image data, holdthe image data, and output the image data to the light emittingelements.
 3. An image forming apparatus as claimed in claim 2, beingcharacterized in that there are lines of pixels to be exposed and linesof pixels not to be exposed on said image carrier, the light emittingelements on the respective lines are arranged to correspond to the linesof pixels to be exposed, respectively, said storage means are arrangedto correspond to both the lines of pixels to be exposed and the lines ofpixels not to be exposed, respectively, and the storage meanscorresponding to the lines of pixels not to be exposed do not outputsaid image data.
 4. An image forming apparatus as claimed in claim 3,being characterized in that the interval in the sub scanning directionbetween spot positions formed on the image carrier by said lightemitting elements is an integral multiple of the pixel pitch in the subscanning direction.
 5. An image forming apparatus as claimed in any oneof claims 1 through 4, being characterized in that said light emittingelements are controlled by a driving circuit according to the activematrix method.
 6. An image forming apparatus as claimed in claim 5,being characterized in that the amounts of light of said light emittingelements are controlled in the PWM method.
 7. An image forming apparatusas claimed in claim 5, being characterized in that the amounts of lightof said light emitting elements are controlled in the intensitymodulation method.
 8. An image forming apparatus as claimed in any oneof claims 1 through 4, being characterized in that each of said lightemitting elements comprises an organic EL.
 9. An image forming apparatusas claimed in any one of claims 1 through 4, being characterized in thatthe image forming apparatus is of a tandem type which comprises at leasttwo image forming stations each having an image carrier and furtherhaving a charging means, an exposure head, a developing means, and atransfer means which are arranged around said image carrier and forms acolor image by passing a transfer medium through the respectivestations.
 10. An image forming method using a plurality of lines each ofwhich has a plurality of light emitting elements and which are arrangedto have rows in the sub scanning direction of an image carrier and usingstorage means, designed to transport image data formed by control means,hold the image data, and output the image data to the light emittingelements, which are arranged to correspond to the lines of the lightemitting elements, respectively, said image forming method beingcharacterized by comprising a step of actuating the light emittingelements on the first line to expose pixels on the image carrier tolight according to the image data outputted from the storage means, astep of moving the image carrier for a pixel pitch, a step oftransmitting the image data to the storage means for the next linesynchronously with the movement of the image carrier in timing, and astep of actuating the light emitting elements on the next line to emitthe same amount of light as that of the light emitting elements on theformer line to repeatedly expose said pixels so that said pixels aresubjected to multiple exposure by the light emitting elements on therespective lines by transmitting the image data by the storage meanswith moving the image carrier for the pixel pitch.
 11. An image formingmethod as claimed in claim 10, being characterized by further comprisinga step of actuating the light emitting elements according to thegradation output formed by said control means to expose the pixels. 12.An image forming apparatus in which a plurality of lines each having aplurality of light emitting elements are arranged to have rows in thesub scanning direction of an image carrier so that light emittingelements are arranged in a matrix in a plane, wherein pixels on saidimage carrier are exposed by the light emitting elements aligned in oneline and exposed again by the light emitting elements aligned in thenext line after the movement of said image carrier, and in the samemanner, said pixels are sequentially exposed by the light emittingelements on another line after the movement of said image carrier so asto achieve multiple exposure of the pixels, said image forming apparatusbeing characterized by comprising storage means for storing informationof misalignment of the mounted position of the line head relative to theapparatus, light emitting elements for adjusting the image positionwhich are preliminarily arranged in respective lines of said line head,and control means for inserting blank data to the image data for everyline of the light emitting elements corresponding to the misalignment soas to form image in normal position by correcting said misalignment ofthe mounted position of the line head according to the information ofmisalignment of the mounted position of the line head.
 13. An imageforming apparatus comprising line heads in which a plurality of lightemitting elements are arranged in a matrix in a plane to form aplurality of unicolor images to be superposed on each other, storagemeans for storing information of misalignment of the mounted position ofthe line head relative to the apparatus, and control means for insertingblank data to the image data for every line of the light emittingelements corresponding to the misalignment so as to form image in normalposition by correcting said misalignment of the mounted position of theline head according to said stored information of misalignment of themounted position of the line head.
 14. An image forming apparatus asclaimed in claim 13, being characterized in that the image formingapparatus is of a tandem type which comprises at least two image formingstations each having an image carrier and further having a chargingmeans, an exposure head, a developing means, and a transfer means whichare arranged around said image carrier and forms a color image bypassing a transfer medium through the respective stations.
 15. An imageforming method using a plurality of lines each of which has a pluralityof light emitting elements and which are arranged to have rows in thesub scanning direction of an image carrier and using storage means,designed to transport image data formed by control means, hold the imagedata, and output the image data to the light emitting elements, whichare arranged to correspond to the lines of the light emitting elements,respectively, said image forming method being characterized bycomprising a step of preliminarily arranging light emitting elements foradjusting the image position respective lines of said line head, a stepof storing information of misalignment of the mounted position of theline head relative to the apparatus, a step of inserting blank data tothe image data for every line of the light emitting elementscorresponding to the misalignment so as to form image in normal positionby correcting said misalignment of the mounted position of the line headaccording to said stored information of misalignment of the mountedposition of the line head, a step of actuating the light emittingelements on the first line to expose pixels on the image carrieraccording to the image data outputted from the storage means, a step ofmoving the image carrier for a pixel pitch, a step of transmitting theimage data to the storage means for the next line synchronously with themovement of the image carrier in timing, and a step of actuating thelight emitting elements on the next line to emit the same amount oflight as that of the light emitting elements on the former line torepeatedly expose said pixels.
 16. An image forming method as claimed inclaim 15, being characterized by further comprising a step of actuatingthe light emitting elements according to the gradation output formed bysaid control means to expose the pixels.
 17. An image forming apparatuscomprising a line head in which a plurality of lines each having aplurality of light emitting elements aligned in the main scanningdirection are arranged to have rows in the sub scanning direction of animage carrier so that light emitting elements are arranged in a matrixin a plane, wherein pixels on said image carrier are exposed by thelight emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements on another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, said image forming apparatus being characterized by comprisingstorage means for storing information of tilt of the line head relativeto the main scanning direction, image data supplying means for supplyingimage data to the respective light emitting elements, delaying means fordelaying the timing of supplying image data from said image datasupplying means to the light emitting elements, and control means forconducting delay control to the image data to be supplied from saiddelaying means to light emitting elements according to said informationof tilt in such a manner that the position of image formationcorresponding to pixels on the image carrier is corrected from the tiltof the line head.
 18. An image forming apparatus as claimed in claim 17,being characterized in that said light emitting elements are dividedinto a plurality of blocks and the delay control is conducted to imagedata to be supplied to said light emitting elements for every block. 19.An image forming apparatus as claimed in claim 17 or claim 18, beingcharacterized in that a plurality of said line heads are arranged tocorrespond to different colors, respectively, and the light emittingelements of the line head which is tilted is subjected to said delaycontrol during multiple exposure in which the respective colors aresuperposed on each other.
 20. An image forming apparatus as claimed inclaim 17 or claim 18, being characterized in that, among said pluralityof light emitting element lines, the first light emitting element lineis controlled with a delay control signal for correcting the tilt ofsaid line head and the light emitting element lines including and afterthe second light emitting element line are controlled with signalsformed by adding signal corresponding to the timing shift from theformer light emitting element line to the aforementioned delay controlsignal for the front light emitting element line.
 21. An image formingapparatus as claimed in claim 17 or claim 18, being characterized inthat said storage means is disposed in the apparatus body.
 22. An imageforming apparatus as claimed in claim 17 or claim 18, beingcharacterized in that said storage means is disposed in a cartridge inwhich the line head is arranged.
 23. An image forming apparatus asclaimed in claim 17 or claim 18, being characterized in that saidstorage means is disposed in the line head.
 24. An image formingapparatus as claimed in claim 17 or claim 18, being in characterized inthat said light emitting elements are controlled by a driving circuitaccording to the active matrix method.
 25. An image forming apparatus asclaimed in claim 17 or claim 18, being characterized in that the amountsof light of said light emitting elements are controlled in the PWMmethod.
 26. An image forming apparatus as claimed in claim 17 or claim18, being characterized in that the amounts of light of said lightemitting elements are controlled in the intensity modulation method. 27.An image forming apparatus as claimed in claim 17 or claim 18, beingcharacterized in that each of said light emitting elements comprises anorganic EL.
 28. An image forming apparatus as claimed in claim 17 orclaim 18, being characterized in that the image forming apparatus is ofa tandem type which comprises at least two image forming stations eachhaving an image carrier and further having a charging means, an exposurehead, a developing means, and a transfer means which are arranged aroundsaid image carrier and forms a color image by passing a transfer mediumthrough the respective stations.
 29. An image forming apparatuscomprising a line head in which a plurality of lines each having aplurality of light emitting elements aligned in the main scanningdirection are arranged to have rows in the sub scanning direction of animage carrier so that light emitting elements are arranged in a matrixin a plane, wherein pixels on said image carrier are exposed by thelight emitting elements aligned in one line and exposed again by thelight emitting elements aligned in the next line after the movement ofsaid image carrier, and in the same manner, said pixels are sequentiallyexposed by the light emitting elements on another line after themovement of said image carrier so as to achieve multiple exposure of thepixels, said image forming apparatus being characterized by comprisingstorage means for storing information of tilt of the line head relativeto the main scanning direction, and control means for controlling lightemitting elements which protrude from the normal exposure line, amonglight emitting elements aligned in the sub scanning direction of saidline head, to emit smaller amount of light and controlling the imagedata to be supplied to light emitting elements in such a manner that theposition of image formation corresponding to the pixels on the imagecarrier is corrected from the tilt of the line head.
 30. An imageforming method using a line head in which a plurality of light emittingelement lines each having a plurality of light emitting elements alignedin the main scanning direction are arranged in a matrix in a plane tohave rows in the sub scanning direction of an image carrier so thatpixels on the image carrier are repeatedly exposed by light emittingelements on the respective lines to achieve the multiple exposure, saidimage forming method being characterized by comprising a step of storingthe information of tilt of said line head relative to the main scanningdirection and a step of controlling the image data to be supplied tolight emitting elements by delaying the supply timing in such a mannerthat the position of image formation corresponding to the pixels on theimage carrier is corrected from the tilt of the line head.
 31. An imageforming method using a line head in which a plurality of light emittingelement lines each having a plurality of light emitting elements alignedin the main scanning direction are arranged in a matrix in a plane tohave rows in the sub scanning direction of an image carrier so thatpixels on the image carrier are repeatedly exposed by light emittingelements on the respective lines to achieve the multiple exposure, saidimage forming method being characterized by comprising a step of storingthe information of tilt of said line head relative to the main scanningdirection and a step of controlling light emitting elements whichprotrude from the normal exposure line to emit smaller amount of light,wherein the image data supplied to light emitting elements arecontrolled such that the position of image formation corresponding tothe pixels on the image carrier is corrected from the tilt of the linehead.